Apply suggestions from code review

Co-authored-by: Martijn Laarman <[email protected]>

Author: reakaleek

src/Elastic.Documentation/Legacy/ILegacyUrlMapper.cs

@@ -11,7 +11,7 @@ public record LegacyPageMapping(string Url, string Version)
 
 public interface ILegacyUrlMapper
 {
-	List<LegacyPageMapping> MapLegacyUrl(IReadOnlyCollection<string>? mappedPages);
+	IReadOnlyCollection<LegacyPageMapping> MapLegacyUrl(IReadOnlyCollection<string>? mappedPages);
 }
 
 public record NoopLegacyUrlMapper : ILegacyUrlMapper

src/tooling/docs-assembler/AssembleSources.cs

@@ -40,7 +40,7 @@ public class AssembleSources
 
 	public FrozenDictionary<Uri, TocTopLevelMapping> TocTopLevelMappings { get; }
 
-	public FrozenDictionary<string, List<string>> HistoryMappings { get; }
+	public FrozenDictionary<string, IReadOnlyCollection<string>> HistoryMappings { get; }
 
 	public FrozenDictionary<Uri, TocConfigurationMapping> TocConfigurationMapping { get; }